This invention relates to a process for converting nonradioactive hazardous chemical wastes into inert, nonleachable glass billets.
A major problem which exists in modern industry is the generation of liquid hazardous waste. This is particularly true in the production of semiconductors, plastics and in the metal refining, electroplating, petroleum and other industries. Current Environmental Protection Agency regulations prohibit the landfilling of liquid hazardous wastes after Jan. 1, 1986, thus eliminating the major current disposal technology. Other methods for the disposal of hazardous wastes include incineration and solidification in cement. The cement solidification technology is expensive and increases the waste volume. Incineration processes do not readily lend themselves to the treatment of materials which form hazardous inorganic oxides since these materials would remain as ash after the incineration process. This is particularly true when the waste material contains heavy or toxic metals such as lead, mercury, cadmium, beryllium, nickel, vanadium and others. The particular difficulty with such metallic pollutants is their stability. While organic substances are degradable by natural processes or by pyrolysis, metals are not degradable. The residue from the pyrolysis of metallics can be leached into groundwater by natural chemical reactions and thereby enter the food chain.
Consequently there is a need for a process which can be used to convert hazardous wastes to materials which can be safely and conveniently landfilled.
A method which has been used to dispose of radioactive liquid wastes is the formation of solid bodies which incorporate such wastes. For example, U.S. Pat. No. 3,321,409 of Grover et al. (1967) is directed to a process for producing a fused glass which contains radioactive waste oxides. This process involves introducing into a container an aqueous solution containing the radioactive wastes. Solid, substantially dry particles consisting of glass forming materials are separately fed into the container. The container is heated to remove the water with continued heating to effect complete fusion of the glass forming material and the radioactive waste into a leach resistant glass.
U.S. Pat. No. 4,020,004 of Schulz et al. (1977) is directed to the immobilization of complex radioactive cesium ferrocyanides by incorporating these materials in a dense insoluble glass. The waste materials are mixed with the glass forming constituents in a dry state and then melted and allowed to solidify. It is disclosed that the ferrocyanides must be fused with sodium carbonate and a mixture of (a) basalt and boron trioxide or (b) silica and lime.
Scheffler et al. in U.S. Pat. No. 4,297,304 (1981) disclose a number of disadvantages with the processes described in the foregoing patents. During the high temperature stages of these processes, significant quantities of radioactive substances evaporate from the not yet solidified waste. Complicated filters, gas washing columns and condensate separators are required to recapture these materials. Also there are troublesome corrosion problems encountered during the melting of the glass. To overcome these problems, Scheffler et al. remove most of the water to form a concentrate which is then combined with a clay like substance by kneading the mixture. The resulting mixture is formed into a molded body which is calcined at temperatures up to about 800.degree. C. and then fired at temperatures between 800.degree. and 1400.degree. C. to form practically undissolvable mineral phases. The molded body is enclosed on all sides in a dense, continuous ceramic or metallic matrix. This process has the disadvantage of requiring multiple steps and the transfer of the waste materials from a concentrator to a mixer and subsequently to the final calcining device.
U.S. Pat. No. 4,376,070 of Pope et al. (1983) discloses that a problem exists in mixing oxide waste with glass forming constituents. It is taught that phase separation can occur during the melting process resulting in a heterogeneous glass which is lacking in chemical durability. The process disclosed in this patent attempts to solve this problem by partially hydrolyzing the glass formers, for example silicates, and then mixing these materials with the nuclear waste. The resulting mixture is heated to remove any organic materials and water and dry the composition. The dried composition is then melted and cooled to form glass.
U.S. Pat. No. 4,424,149 of Bege et al. (1984) discloses a method for disposing of radioactive borate containing liquids and ion exchange resins. This patent discloses that the wastes are preferably concentrated by predrying prior to adding glass forming materials although the wastes can be mixed directly with glass formers in liquid form. If the liquid waste mix is fed directly to a melter, first the water evaporates and then the solid components are melted. The organic materials such as the ion exchange resins burn leaving a dry residue and which is incorporated in the glass melt. It is also disclosed that the melting process can be carried out in steps, i.e. a charge of the glass forming composition can be fed into a vessel and then melted. This procedure is repeated with additional charges until the vessel is filled to the desired height with melt. The waste gases evolved during the melting process can be purified by passage through a gas washer and/or a filter. A preferred apparatus for performing the melting operation is an electric glass melting furnace with a tight enclosure.
These references collectively teach that a vitrification process must be specifically adapted to the particular waste material which is being treated. Also the processes disclosed are complex, batch operations in which the cost can only be justified when dealing with small quantities of radioactive materials that can not be effectively treated in any other manner.
Applicants have found a vitrification process which can effectively and efficiently treat nonradioactive wastes to produce nonleachable glass matrices which are suitable for landfill disposal. In addition, the processes and apparatus disclosed herein permit continuous operation. The continuous operation provides an economical method of treating hazardous inorganic oxide-forming wastes. This is in contrast with the batch operations disclosed in the patents cited hereinabove.